1. Field of the Invention
This invention relates in general to wireline logging of oil wells, and in particular to a method of pulsed neutron logging for determining earth formation characteristics.
2. Description of the Prior Art
In pulsed neutron logging, a downhole instrument is lowered into a well on a conductor cable. The downhole instrument has a pulsed neutron source and two gamma ray detectors spaced above the neutron source. As the downhole instrument is pulled slowly upward, the neutron source is pulsed or fired to cause a cloud of neutrons to be emitted to irradiate the formation. The neutrons are slowed down by collisions in the formation, particularly by hydrogen if present. Once they reach a state known as thermal neutrons, some will be captured. Chlorine nuclei capture the neutrons much more readily than other elements present. Once captured, the nuclei become excited, and emit gamma rays for a short period of time. The gamma rays are detected by the two scintillation counters.
A pulse of neutrons will be followed by a time period in which the gamma rays are received by the detectors. The rate at which the counts per second decay is a function of the formation thermal decay time. The ratio of the two detectors can be used to calculate the porosity, while the formation thermal decay time can be used to calculate other earth formation characteristics, such as the water saturation.
The decay rate is exponential down to a constant background rate. The background rate is the count rate that exists due to gamma rays being detected by the counter that did not emanate as a result of neutron capture. This background rate depends upon the formation and to a large extent on the downhole tool itself, which becomes activated by the neutron radiation.
Different methods are used to compute earth formation characteristics from the decay rate curve. An equation can be used to express the decay rate curve, but it will have unknown variables which include the background count rate, the formation lifetime or decay constant, and at a time equal zero, the number of excited nuclei or the count rate. One technique divides the sampling time following each pulse into time gates, then assumes that the last one or two time gates in the sampling period represent count rates due only to background. A second technique periodically turns off the neutron source for a period in which the background is measured. In both cases, once the background is known, the other variables are computed using various techniques. Both, however, make assumptions of the background, which can lead to errors.